The present invention relates to the manufacture of semiconductor-based products. More particularly, the present invention relates to improved focus rings in plasma processing systems and methods therefor.
The use of plasma-enhanced processes in the manufacture of semiconductor-based products (such as integrated circuits or flat panel displays) is well known. Generally speaking, plasma-enhanced processes involve the processing of a substrate (e.g., a glass panel or a semiconductor wafer) in a plasma processing chamber. Within the plasma processing chamber, a plasma may be ignited out of appropriate etchant or deposition source gases to respectively etch or deposit a layer of material on the surface of the substrate.
To facilitate discussion, FIG. 1 illustrates a simplified inductively coupled plasma processing system, representing a suitable plasma processing system for performing plasma-enhanced processes on substrates. To simplify the illustration, FIG. 1 as well as the figures herein has not been drawn to scale. It should be borne in mind, however, that although an inductively-coupled plasma processing system is discussed in detail in this disclosure, the invention disclosed herein may be employed in any known plasma processing system, including processing systems adapted for deposition, cleaning, and/or etching. With respect to etching systems, the invention may be employed in, for example, inductively-coupled plasma etching, dry etching, reactive ion etching (RIE), magnetically enhanced reactive ion etching (MERIE), electron cyclotron resonance (ECR) etching, or the like. Note that the above is true irrespective of whether energy to the plasma is delivered through capacitively coupled parallel electrode plates, through ECR microwave plasma sources, or through inductively coupled RF sources such as helicon, helical resonators, and coil arrangements (whether planar or nonplanar). ECR and inductively coupled plasma processing systems, among others, are readily available commercially. Inductively coupled plasma systems, such as the TCP.TM. brand inductively coupled plasma systems, are available from Lam Research Corporation of Fremont, Calif.
Referring now to FIG. 1, a plasma processing system 100 includes a plasma processing chamber 102. Above chamber 102, there is disposed an electrode 104, which is implemented by a coil in the example of FIG. 1. Electrode 104 is energized by a radio frequency (RF) generator 106 via a conventional matching network 108. In the example of FIG. 1, RF generator 106 sources RF energy having a frequency of about 13.56 MHz although other appropriate frequencies may also be employed.
Within plasma processing chamber 102, there is shown a shower head 110, representing the gas distribution apparatus for releasing gaseous etchant or deposition source gases into a region 112 between itself and a substrate 114. Substrate 114 is introduced into plasma processing chamber 102 and disposed on a substrate-holding chuck 116, which may be implemented as an electrostatic (ESC) chuck (either monopolar or bipolar in configuration). Chuck 116 may also be a mechanical chuck, a vacuum chuck, or simply a workpiece holder. Chuck 116 acts as the second electrode and is biased by a radio frequency (RF) generator 118 via a matching network 120. Like RF generator 106, RF generator 118 of the example of FIG. 1 also sources RF energy having a frequency of about 13.56 MHz although other suitable frequencies may be employed as well.
To facilitate plasma-enhanced processing, the etchant or deposition source gas is flowed through the shower head 110 and ignited by the RF energy supplied by RF generators 106 and 118. During plasma-enhanced processing, the by-product gases are exhausted out of chamber 102 through exhaust port 122 (using an appropriate turbo pump arrangement). After plasma-enhanced processing is completed, substrate 114 is removed from plasma processing chamber 102 and may undergo additional processing steps to form the completed flat panel display or integrated circuit.
In FIG. 1, a focus ring 124 is also shown. In the example of FIG. 1, a portion of focus ring 124 underlies substrate 114 and overlaps a portion of substrate-holding chuck 116. As is well known to those familiar with the plasma processing art, the focus ring helps focus the ions from the RF-induced plasma region 112 onto the surface of substrate 114 to improve process uniformity, particularly at the edge of the substrate. This is because when RF power is supplied to substrate-holding chuck 116 (from radio frequency generator 118), equipotential field lines are set up over substrate 114 and focus ring 124. These field lines are not static but change during the RF cycle. The time-averaged field results in the bulk plasma being positive and the surface of 114 and 116 negative. Due to geometry factors, the field lines are not uniform at the edge of substrate 114. The focus ring helps direct the bulk of the RF coupling through substrate 114 to the overlying plasma by acting as a capacitor between the plasma and the powered electrode (e.g., RF-powered chuck 116).
During plasma processing, the positive ions accelerate across the equipotential field lines (shown representatively in FIG. 1 as equipotential field lines 130) to impinge on the surface of substrate 114, thereby providing the desired processing effect (such as deposition or anisostropic etching). Although ion acceleration and impact upon substrate 114 are generally desirable if properly controlled, such ion acceleration and impact upon focus ring 124 tend to unduly erode focus ring 124. In the prior art, focus ring erosion is typically thought to be unavoidable. In the prior art, most of the attention is directed toward finding ways to minimize the effect such erosion causes (e.g., particulate contamination) on the process. By way of example, system designers in the prior art may form focus ring 124 out of a material generally similar to that employed to form the walls of plasma processing chamber or substrate 114 so that erosion does not introduce a different type of particulate contamination into the chamber. A popular material for use in forming focus ring 124 in the prior art is aluminum oxide (Al.sub.2 O.sub.3).
As is known, however, aluminum oxide is a material with a relatively high dielectric constant, i.e., having relatively low impedance. Because of this, a relatively high potential difference exists between the upper surface 134 of focus ring 124 and the plasma sheath. This potential difference manifests itself by the presence of multiple equipotential field lines 130 along upper surface 134 of focus ring 124. The presence of multiple equipotential field lines over upper surface 134 causes the ions from RF-induced plasma region 112 to impinge with a relatively high level of force on upper surface 134 of focus ring 124 since ions tend to accelerate across equipotential field lines in a direction orthogonal to the field lines themselves.
Ion impact on upper surface 134 causes, in addition to the aforementioned contamination problem, other undesirable consequences. For example, if enough of focus ring 124 is eroded away by the impinging ions, the plasma material may begin to attack underlying chuck 116, which may give rise to more (and a different type of) particulate contamination and may eventually necessitate the replacement of chuck 116. Further, if chuck 116 is an electrostatic (ESC) chuck (i.e., a chuck that depends on electrostatic forces to clamp substrate 114 to its upper surface), current leakage to the plasma from the chuck (due to plasma directly contacting chuck 116 through eroded focus ring 124) may alter the ability of the ESC chuck to clamp substrate 114 to it. With inadequate clamping, the substrate may pop off the chuck during plasma processing or there may be inadequate heat transfer between the substrate and the chuck to ensure reliable processing results.
In view of the foregoing, there are desired improved techniques for reducing focus ring erosion in a plasma processing chamber.